In the manufacture of vacuum electron tubes, it is a common practice to provide an envelope having a small glass exhaust tubulation. During the manufacture of the tube, the envelope is exhausted of gases through the tubulation. Then, the tubulation is heated to melt the glass at a location that is close to the external surface of the envelope. The molten glass necks in, closing the path therethrough, and thereby seals the interior of the envelope from the ambient. This sealing of the tubulation is referred to in the art of "tipping-off".
In many types of tubes, including most cathode-ray tubes, the exhaust tubulation is integral with the stem of the tube. The stem includes a glass disc on wafer having a circular array of electrically-conducting leads sealed into, and extending out from, the wafer, with the exhaust tubulation usually disposed centrally of, and within the array of, the leads. The glass of the wafer is usually thicker adjacent each lead, which thickening is referred to as a "fillet".
A typical heating unit used for tipping-off exhaust tubulations comprises a cylindrical electrical resistance heater coil sized to fit symmetrically around the outside of the circular array of leads, with the stem of the tube resting on the top of the endwall of the unit. One type of heater unit, described in U.S. Pat. No. 3,002,076 issued Sept. 26, 1961 to M. K. Massey, includes separate metal straps that function as heat shields between each lead and the heater coil in a circular array. Factory use of the prior heater units has been satisfactory generally, although an undesirable percentage of tubes has exhibited cracked fillets and/or nonsymmetrical melting of the exhaust tubulation.
U.S. Pat. No. 4,451,725 issued May 29, 1984 to J. Mount reduces the above-mentioned problems by providing a heat shield that includes a unitary body of heat conducting material having a hollow, cylindrical main body with an inwardly-extending flange at one end and an outwardly-extending flange at the other end thereof. The wall of the main body is shorter than the length of the longest lead so that the longest lead, or leads, rests on the lower, inwardly-directed flange. The lower flange has an aperture therethrough that is large enough for the tubulation to pass through but smaller than the circle of leads through the stem. A drawback of this latter structure is that the lower flange which contacts one or more leads, tends to bend the leads, and the lead-supporting heat shield requires a great deal of electrical power to develop a good tip-off. Additionally, as the heat shield oxidizes, holes develop therein which create hot-spots in the tubulation and asymmetric seals which have unequal stress and are subject to failure. Another drawback of the patented structure is the cylindrical ceramic heater retainer, which is located concentrically around the coil, is a separate structure that is free to move and thus permits an outward movement of the coil which results in non-symmetric heating of the tubulation. Accordingly, a need exists for a heating unit which is free from the sealing problems encountered in the prior structures.